Many existing electronic ballasts for powering gas discharge lamps include a self-oscillating half-bridge type inverter. An essential part of a self-oscillating half-bridge inverter is an inverter startup circuit that initiates self-oscillating operation of the inverter.
FIG. 1 describes a common prior art ballast 10 that includes a front-end circuit 100, a self-oscillating half-bridge inverter 200, and a series resonant output circuit 300. Front-end circuit typically consists of a full-wave rectifier circuit followed by an appropriate circuit (e.g., a boost converter, a valley-fill circuit, etc.) for providing a substantially direct current (DC) voltage, VRAIL, and (optionally) for providing power factor correction. Self-oscillating half-bridge inverter 200 receives the substantially DC voltage, VRAIL, via inverter input terminals 202,204) and provides, via inverter output terminals 206,208, a high frequency output voltage to series resonant output circuit 300. Series resonant output circuit 300 provides a high voltage for igniting lamp 30 and a magnitude-limited current for operating lamp 30 at a desired current.
As illustrated in FIG. 1, self-oscillating half-bridge inverter 200 includes a diac-based inverter startup circuit 270,274,276,280 for initially activating inverter transistor 240. The diac-based inverter startup circuit consists of a resistor 270, a capacitor 274, a diac 276, and a diode 280. The structure and operation of the diac-based starting circuit is well known to those skilled in the arts pertaining to power supplies and electronic ballasts.
Once inverter transistor 240 is initially turned on by inverter startup circuit 270,274,276,280, inverter transistors 220,240 are complementarily commutated (i.e., switched on and off such that, when transistor 220 is on, transistor 240 is off, an vice-versa) by drive signals provided by a base-drive transformer comprising a primary winding 340 and two secondary windings 342,344.
Inverter startup circuits that require voltage breakdown devices such as diacs are known to be plagued by problems relating to cost and reliability. Accordingly, a need exists for an improved inverter startup circuit that does not require a voltage breakdown device such as a diac. A further need exists for an improved inverter startup circuit that is less costly and more reliable than a diac-based inverter startup circuit. Accordingly, an electronic ballast with such an improved inverter startup circuit would represent a significant advance over the prior art.